1. Field of the Invention
The present invention relates generally to methods and systems for detecting the position of a moving object relative to a path upon which the moving object is being controllably steered using reflected light. More particularly, the present invention relates to methods and systems for detecting a lateral position of a moving vehicle (e.g., an automobile), relative to a road surface ahead of the vehicle so that steering control signals required to maintain path alignment can be anticipated.
2. State of the Art
In the past, light sensing systems have been used for determining the position (e.g., lateral position) of moving objects, such as automobiles along a designated path. One such known system is described in U.S. Pat. No. 4,702,240, and uses an optical sensor to detect light reflected from a reflective portion of the road surface.
More particularly, this patent discloses illuminating an area immediately beneath a vehicle. A series of photocells are provided to detect the reflected beam. These reflections are transformed into data which represents derivation of the vehicle from a desired path. This data is thus used to control vehicle steering and realign the vehicle on the path. Such a system suffers from the disadvantage that it only scans an area immediately beneath the vehicle to derive reflection data. Accordingly, the system is unable to anticipate upcoming changes in the road contour fast enough for vehicle steering to be automatically controlled.
U.S. Pat. No. 4,049,961 relates to an automatic guidance system for an automobile wherein lasers are used to scan a limited region ahead of the automobile. However, this system merely indicates the deviation of the vehicle position from an imaginary centerline of the roadway. Thus, the system disclosed in U.S. Pat. No. 4,049,961 is unable to determine road curvature in response to reflected signals and is therefore not able to anticipate required steering control in response to changing road geometry.
In co-pending U.S. application Ser. No. 07/592,235, filed Oct. 3, 1990, a moving oscillating reflector is disclosed for sweeping a light source back and forth across a given field of view forward of vehicle travel However, such moving parts are susceptible to damage (e.g., due to vibration) and/or fatigue in relatively hostile environments, such as automobiles.
U.S. application Ser. No. 07/592,235 further discloses an alternate embodiment of a light detecting system for use in determining lateral position of a moving object. One example of such a system is shown in FIG. 1. The FIG. 1 system includes a stationary light source 2, such as a laser, for illuminating a predetermined field of view. The field of view is selected to include reflectors 4, 6 and 8 situated in advance of the vehicle's direction of movement.
Light reflected by the reflectors 4, 6 and 8 is received by a light detector 10 via a lens 9. The light detector 10 includes, for example, 20 to 35 PIN diodes or avalanche photo-diodes (APD's). A corresponding number of bias power supplies are represented generally as element 12, and a corresponding number of pre-amplifiers are represented generally as element 14. A separate bias power supply and pre-amplifier are typically required for each diode of the detector array 10.
Outputs from each pre-amplifier are applied to a multiplexor 16 which is controlled by clock 18. Multiplexor 16 sequentially gates amplified data received by a selected one of the diodes to a range counter 20. The output 22 of the range counter 20 represents the vehicle distance to a selected reflector. Further, light received by the range counter is directed to an angle detector 24 for determining an angular orientation of the reflected light.
In operation, pulsed light from the light source 2 is reflected by one or more of the reflectors. Reflected light is focused by lens 9 onto detector array 10. The output associated with a selected diode in detector array 10 is then passed via the multiplexor 16 to the range counter 20.
The range counter 20 is a digital counter which counts the number of clock pulses that occur between the time a light pulse from light source 2 is transmitted and received by one of the diodes in the detector array 10. Such information represents the range of the vehicle to a given reflector. Further, by identifying the diode in the detector array which received the reflected light pulse, the angle of the reflected light can be readily ascertained.
Thus, the FIG. 1 system can provide lateral position information based on detected range and angle data. However, the FIG. 1 system involves a plurality of photodiodes, bias power supplies, preamplifiers and a multiplexor. This arrangement results in relatively high production costs.
Further, not only does the use of a multiplexor increase system cost and complexity, but in addition, the multiplexor can potentially introduce detection inaccuracies. More particularly, the multiplexor selects the output of a single diode as an input to the range counter. Therefore, when a reflected light pulse is received by a given diode, the output of that diode must be selected by the multiplexor.
To permit accurate control of a vehicle along a desired path, reflectors should be sensed at a distance of, for example, 50 feet in advance of the vehicle. Because an exemplary light source pulse has a pulse width of 5 to 20 nanoseconds (ns) and will travel at a rate of one foot per nanosecond (ft/ns), a reflected light pulse will typically be detected approximately 100 ns after transmission. However, a conventional multiplexor requires 300 nanoseconds (ns) to scan an array of diodes as described above.
Thus, there is a possibility that a received light pulse will dissipate in a diode of the detector array prior to the point in time at which the multiplexor accesses the diode. This detection problem can be associated with a lack of charge storage in the detector array. More particularly, because the same detector array is used for providing both range data (i.e., fast detection) and angle data (i.e., slow detection), outputs of the detector array can not be latched.
Further, while a relatively small size system would be desirable for implementation, an ability to reduce the size of the foregoing light detection system is limited. For example, the optical detector array 10 is relatively large in size (i.e., 20 to 35 detectors), resulting in an array which is one to two inches long. This relatively large detector array is required to obtain a desirable aspect ratio (e.g., field of view 10.degree. wide and 4.5.degree. high).
In addition, diode detectors as described above include insulating spacers between each photodiode, these spacers being approximately 30 micrometers wide. Thus, a relatively large spot of light must be focused onto the detector array. On the contrary, if, for example, a focused light spot having a diameter of 10 micrometers is used, the spot may fall totally within the insulating spacer, thus avoiding detection and resulting in data loss.
The use of a large detector array also requires that the lens 9 have a capability of focusing light along a greater distance. It is desirable to provide a lens having a focal length to diameter ratio that is as close to one as possible (i.e., high speed lens). A relatively large lens (e.g., 7 inches in diameter) would therefore be required to provide a 10.degree. field of view and to focus light on diodes in the one to two inch detector array.
Accordingly, it would be desirable to provide a light sensing system capable of reliably anticipating the contour of a desired path upon which a moving object, such as a vehicle, is being steered. Further, it would be desirable to provide a system capable of sensing the lateral position of the vehicle relative to locations far enough in advance of the vehicle's current position that changes in path contour can be anticipated.